This invention relates to flywheel magnetos with capacitive ignition systems. More particularly, the invention relates to a flywheel magneto having a thyristor controlled capacitive ignition system which includes a flywheel provided with permanent magnets and at least one cooperating coil core.
One known construction of flywheel magnetos having a capacitive ignition system, has arranged on an iron core in the magneto a charging coil which charges a capacitor which immediately on being charged delivers its energy to the primary winding of the ignition coil in the secondary winding of which is induced a high tension ignition voltage. The capacitor is normally built with the remaining non-rotating components of the magneto on an armature plate while the ignition coil is located externally of the magneto and as close to the spark plug of the internal combustion motor it is associated with in order to avoid energy losses as a result of excessive cable lengths.
The energy stored in the capacitor with each revolution of the flywheel is transmitted to the primary winding of the ignition coil through a control means which at a predetermined time, i.e., when ignition is to take place, closes a circuit between the capacitor and the primary winding. The control means may comprise a conventional mechanical switch but normally consists of a trigger coil which is connected with a charging coil and a thyristor which during a starting sequence closes the current circuit between the capacitor and primary winding of the ignition coil.
The charging of the capacitor and the triggering of the thyristor must be very accurately adjusted in order to obtain optimal ignition effects over the whole range of rotational speed, i.e., from start at idling speeds of 400 r.p.m. up to maximum operational speeds of 10,000 r.p.m. The capacitor should obtain a full charge of the charging coil with each separate revolution and only then be discharged by means of the primary winding of the ignition coil. With this sequence of events, no appreciable nor any distributing displacement between the discharging and charging should occur, and most importantly, a triggering sequence should not effect the charging sequence with the capacitor in a manner to cause the charging to be incomplete.
In another flywheel magneto construction, the core comprises a laminated structure of dynamo sheet provided with three pole legs arranged in uniform spaced relationship. The flywheel is provided, for example, with a pole system which includes two permanent magnets whose size and position are adjusted so that the distance between their central lines coincide with the pole distribution of the iron core, while at the same time the distance between the two opposing side surfaces of the permanent magnets is slightly less than the width of the pole legs of the iron core. The charging coil is on the center pole leg of the iron core and a positive charging pulse of relatively high voltage of about 400 volts is generated each time the magnet system is passed, the positive charging pulse being proceeded and followed by negative pulses of low voltage. The negative pulses are blocked by a first diode, while usually a second diode is connected over the charging coil to eliminate excessive blocking voltages on the first diode.
In order to ensure that the capacitor is charged under all circumstances, the starting coil of known flywheel magneto constructions is arranged on the first pole leg of the iron core when viewed in the direction of rotation of the flywheel, i.e. in front of the charging pole. This arrangement provides a wide safety margin with respect to capacitor charging time, since the flywheels which for example, only cause one ignition sequence for each revolution, rotates almost a complete revolution, e.g. about 320.degree., before triggering, i.e. before the capacitor is discharged. On the other hand, such a system presents serious difficulties with regard to the possibilities of eliminating the influence of the triggering sequence on the charging sequence immediately following. These difficulties are caused by the fact that the triggering voltage curve and charging voltage curve, owing to armature reaction are not completely stable, but are liable to intersect each other with increasing revolutions. Thus, the trigger voltage is unable to fall while the thyristor opening voltage is applied before the new charging sequence begins. As a result, the thyristor does not break the circuit to allow the capacitor to recharge. A default circuit braking of the thyristor may cause a serious overloading of the electronic components so that they may be damaged. This means in practice that the ignition system with which the trigger coil is arranged on the first pole leg when viewed in the direction of rotation of the flywheel, will function satisfactorily only if the speed range is restricted to, for example, between 700 and 900 r.p.m.